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Re: under 1 atm vs at 1 atm

I don't know what it means but "at" and "under" mean two different things here. If the measurement was taken "at" 1 atm (whatever that means) then it's exactly "1". If it's "under" 1atm, then it's below 1 atm (so 0.99999 atm or lower).

Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.